The present invention relates to the field of optical fibers. In particular, the present invention relates to an apparatus that provides strain relief for that portion of an optical fiber that is exterior to a surface of a composite structure.
Composite materials, such as graphite/epoxy laminates are rapidly replacing metal and other conventional materials in the manufacture of traditional structures. Normally, such materials are comprised of a variety of layers or plies of a fiber bonded together by a resin or polymer. Such composite structures are extremely strong and lightweight and therefore provide a good strength to weight ratio. Accordingly, these composite structures have gained wide acceptance in many industries, most notably the aircraft industry.
One disadvantage of a composite structure is that when subjected to reoccurring stress, the composite tends to fail catastrophically and in an unpredictable manner. Accordingly, there have been a number of methods and devices that have been utilized to measure the distributed strain in a composite structure.
Most of these techniques incorporate a network of optical fibers embedded between plies within a matrix of a composite structure. The stress which is placed on these fibers causes relative changes in the index of refraction of the fiber which in turn causes a change in the fiber's transmission impedance. In addition, stress placed on the fiber will cause the fiber to back-scatter a portion of optical energy introduced into the fiber. Therefore, an analysis of the composite structure can be performed by launching an optical energy pulse from an optical source, such as a laser, into the optical fibers within the matrix and toward an optical receiver. Alternatively, a reflective surface may be placed at the end of the fiber so that the optical pulse is reflected back to an optical source/receiver. The strain placed on the optical fiber and/or the possible discontinuities within the fiber can then be determined by measuring the elapsed time from the launching of the optical pulse to the reception of the optical pulse. In addition, an analysis of the back-scattering of the optical energy can also be performed.
Optical fiber sensing technology can be used to develop strain and deformation maps of the composite structure by continually monitoring and recording the ability of the optical fibers to transmit optical energy. Similarly, continuous optical monitoring will also detect sudden impacts to the composite structure along with micro-cracks within the structure. Accordingly, the optical fiber systems can be used to generate information on the composite structure throughout its manufacture, installation, maintenance, and use.
A basic concern arises, however, at the point where the optical fiber exits the composite structure. During the manufacture, maintenance, and installation of a composite structure, the fiber may be subjected to a variety of bending forces. An optical fiber can withstand a certain amount of bending, however, such a fiber cannot withstand extreme bending or shearing forces developed at the point of its exit from the composite structure. Therefore, the optical fiber can easily be damaged at points adjacent its exit from the composite structure during the manufacture, installation and maintenance of the composite structure.
A number of references have disclosed a system whereby optical fibers are embedded into a composite material so that the optical fibers may be used in conjunction with a variety of optical devices to monitor the integrity of the composite structure. For example, Lymer et al. U.S. Pat. No. 4,936,649 discloses a damage evaluation system and method using optical fibers embedded within a composite panel. An input bundle of optical fibers is connected to a light source and an output bundle of fibers is connected to an output connector. However, there is no discussion with respect to providing strain relief to the optical fibers as they exit the optical panel.
Similarly, Spillman Jr. U.S. Pat. No. 4,983,034 discloses an optical frequency domain distributed strain sensor for determining the strain distribution along an optical fiber. An optical fiber is embedded within a strain producing structure and is attached to the sensing fiber circuit by a connector. No details are given as to providing strain relief to the fiber as it exits the strain-producing structure.
Therefore, there is a need for an apparatus that provides strain relief for an optical fiber mounted between plies within the matrix of a generally planar laminated composite structure, wherein the fiber extends from a surface of the composite structure.